This chapter examines what individual and social-psychological factors best predict Korean heritage language learners’ maintenance of heritage language (HL) proficiency. The examined predictors are age of immigration, length of Korean language education, degree and frequency of HL use, cultural identity, and motivation. A language background survey and a Korean language proficiency test developed by KICE (Korea Institute of Curriculum and Evaluation) were administered to 120 Korean-American college students. The results of the study indicate that although most of the examined predictors are significantly related to Korean language proficiency, the degree and frequency of HL use is found to be the best predictor of one’s HL proficiency. The participants’ narrative responses on their HL maintenance also reveal that those who maintain a higher level of Korean language proficiency are more highly motivated to maintain Korean, have had more parental involvement in HL maintenance, and use more Korean than those who are not as proficient.